ARACHNID A. 



185 



ders that they represent the antennae of insects and 

 the internal antennag of the decapod Crustacea, but so 

 little attention has hitherto been paid to the compa- 

 rative anatomy of these analogical organs that it 5s 

 perhaps the safest course to regard such analogies 

 as undetermined. In addition to such organs, the 

 month is furnished with a lower lip and a pair of 

 lateral moveable instruments, similar to the lower 

 jaws of the mandibulated insects furnished externally 

 with a pair of jointed appendages or palpi. All the 

 arachnida appear to be carnivorous, but some are 

 parasitic upon the bodies of other animals, and in 

 these the mouth undergoes a considerable change of 

 structure, being composed of an instrument capable 

 of suction, although formed of the same typical 

 limber of organs as the mouth of the preceding. 



The portion of the body, succeeding the cephalo- 

 thorax, constitutes the abdomen ; in general it is soft, 

 more or less globular ; on its under surface are to be 

 observed a certain number of apertures or spiracles, 

 and the anal aperture, as well as the spinnerets, 

 when present, are placed at its posterior extremity. 



The external covering of the arachnida may be 

 regarded as of a leathery rather than a horny texture, 

 but in some species it assumes a considerable degree 

 of rigidity; it forms, however, in all cases an external 

 skeleton, to which the muscles are internally affixed 

 as in insects. 



Of the senses of the arachnida it may be observed 

 that, according to the best of our necessarily imper- 

 fect knowledge of such matters, they appear to pos- 

 sess all the five senses with which the higher animals 

 are endowed ; that the minute simple stemmata on 

 the crown of the metathorax are eyes, and that these 

 animals possess the sense of sight cannot be doubted ; 

 every body havingobserved the hunting spider (salticits) 

 throw itself to a considerable distance upon its prey. 

 Here it is evident that the instinct of the animal, 

 acting upon the impressions produced by the posses- 

 sion of the sense of sight, induces it to do an act for 

 the gratification of another sense, that of taste. In 

 like manner the sense of touch is possessed in an 

 eminent degree by these animals; and although the 

 accuracy of the poet's observation, that the spider 



" Lives in each thread, and feels along the line," 

 has been called in question in a very popular work 

 ( Insect Miscellanies, p. 5), it is evident, from experi- 

 ments which we have made for the express purpose 

 of ascertaining the relative powers of the senses of 

 sight and touch, that the latter is much stronger than 

 (In- former. When a fly is caught in the web of a 

 spider, the latter instantly stretches out its legs in the 

 direction of the captured and struggling fly and feels, 

 for in no other manner can we describe its proceed- 

 iBngs, the nature of the disturbance which is taking 

 place in its web. In the work above alluded to, it is 

 stated, " We have tried numerous experiments, by 

 moving and vibrating the lines of the webs of many 

 species, so as to imitate as nearly as possible the en- 

 trapment of a fly, but in no case have we succeeded 

 in bringing the spider to the spot, because, as we in- 

 ferred, her eyes always detected our attempted de- 

 ception." Now the experiments made by the author of 

 this article have produced a result so contrary to the 

 above, that it is not without a repetition of them that 

 we have ventured to adduce them against the state- 

 ments in question. These experiments were chiefly 

 made upon the Epeira diadema, one of the largest of 

 the British species, which, from its very common 

 occurrence, beautiful markings, and elegantly-con- 



structed web, must have attracted the attention of the 

 most casual observer in the autumn. Now from its 

 size it is evident, that, if the intelligence which these 

 insects obtain respecting the entrapment of a fly, 

 results from the sense of sight, the epeira would make 

 no use of its legs for obtaining such knowledge, 

 and that its eyes alone would be brought into action ; 

 but so far is this from being the case, that we have 

 repeatedly disturbed the lines of the web, within an 

 inch of the spider, and in the direction of its eyes, but 

 in every instance the legs have been instantly put in 

 motion, which has been continued for a considerable 

 time even after. We have ceased the disturbance, in 

 order that, if possible, the spider, by slightly moving 

 the web itself, might set the fly, which it evidently 

 supposed to have been captured, in motion again, so 

 that, by its repeated struggles, it might ultimately fall 

 an easier prey to the inhabitant of the web. But in 

 addition to this fact, it is to be observed, that the 

 den-like retreats of many spiders are so placed that 

 they cannot possibly see any thing which occurs on 

 the web, and yet no sooner is a fly entrapped in any 

 part of the web than the spider instantaneously 

 acquires a knowledge of the fact, evidently by 

 touch, and darts out of its den upon the luckless 

 captive. Thus we frequently find the spider deeply 

 seated in the aperture of a wall, and the web extended 

 to a considerable distance over its surface ; and yet 

 by the tension of the cords composing the net work, 

 the spider is instantly made aware of what occurs on 

 every part of its surface. Two instances are adduced 

 in the work in question in support of the idea, that the 

 eyes, and not the legs, are the instruments of know- 

 ledge to the spider. First, the long-bodied spider 

 (Tetragnatha extcnsci) is noticed as having probably 

 given rise to the popular opinion under review, not- 

 withstanding it huddles its legs into a close bundle, and 

 which is evidently done with the view of making them 

 appear motionless, although the insect does not the less 

 obtain information by their assistance ; and immedi- 

 ately afterwards the long-legged house-spider(P/zo/cws 

 phalangioid.es) is referred to, as giving more counte- 

 nance to the opinion, because it keeps its legs spread 

 out as if to feel the more readily when any thing is 

 caught ; although the more than usual prominence of 

 its eyes is mentioned as showing the superiority of its 

 powers of sight. We have noticed these contradic- 

 tions, because as they occur in a work of great popu- 

 larity, it is advisable that the erroneous impressions 

 which they must necessarily produce ought to be 

 counteracted as early as possible. 



In the majority of the arachnida, a complete and 

 very distinct system of circulation exists. The heart 

 is contained in the abdomen, and in many species of 

 spiders its pulsations are easily to be observed. It 

 consists of a large longitudinal vessel which emits a 

 certain number of arteries, and receives the veins by 

 means of which the sanguineous fluid returns, after 

 having been aerated by the respiratory organs, to be 

 again distributed through the different parts of the 

 body. 



In this class, as in the insects, the sexes are con- 

 stantly distinct, and impregnation Is necessary for 

 the fecundation of the eggs. Several striking pecu- 

 liarities are connected with the act of impregnation, 

 respecting which the most celebrated physiologists are 

 at variance, and though the pages of the present work 

 can hardly be considered a proper field for discus- 

 sions connected with this subject, i t may be proper 

 to state that it was considered onq of such import- 



